Disclaimer: Yes I know Insider Builds aren't going to be supported, I'm just posting this as a bug report and to help others.
In the last three builds of Windows 10: 14942, 14946, 14951, ShadowFX has broken my Windows install in that when the ShadowFX service loads, dwm.exe crashes, crashing that user session to end, then dwm restarts, crashes, restarts, indefinitely, making the system unusable.
During Windows upgrades this happens at the 76% point, immediately killing the setup session and spawning into the dwm crashes. When I disabled the ShadowFX service, the upgrade was successful but the problem reappeared by starting the service.
I tested this in a Hyper-V virtual machine and the problem did not occur there. It may only affect certain video cards/drivers.
Video card: AMD Radeon HD 5700
Driver: 15.201.1151.1008, 11/4/2015 (installed automatically from Windows Update)